Frequency discriminator



Jan. 23, 1951 s. I. RAMBO 2,539,204

FREQUENCY DISCRIMINATOR Filed Sept. 12, 1946 reguency rag a/7c; F1; 1.fly 2.

flay Mendy my. a.

WITNESSES: INVENTOR ATTORNEY Patented Jan. 23, 1951 FREQUENCYDISCRIMINATOR Sheldon I. Rambo, Baltimore, Md., assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., a. corporation ofPennsylvania Application September 12, 1946, Serial N 0. 696,569

3 Claims.

My invention relates to electronic tube circuits and, in particular,relates to such a circuit adapted to produce a continuous currentvoltage having a magnitude which is substantially proportional, in valueand sign, to the amount by which the frequency of one of two alternatingvoltages differs from that of another alternating voltage.

.Such devices are :irequently known as discriminators, or more preciselyfrequency discriminators, and several diiferent circuits capable ofproducing such output voltages are known in the art. These arrangementsmay be used for varying the tuning or other frequency determiningelements of one circuit to correct any departures of its prior artdiscriminators and is much simpler, cheaper and involves asmallernumber-of tubes and circuit elements.

One object of :myinvention is, accordingly, to produce a discriminatorwhich employs much simpler circuits than those of the prior art.

Another object of my invention is to provide a discriminator having apower output which canbe made relatively large without the employmentofcascaded amplifier tubes and the like.

Another object of my invention is to employ the properties of current inthe plate or grid circuits of a three-electrode tube for frequencydiscrimination between two alternating voltages respectively impressedon a pair of electron-collecting electrodes of the tube.

Other objects of vmy invention will become apparent upon reading thefollowing description, taken in-connection with the drawing, in which:

Figure l isa graph representing the variation of the grid current in athree-electrode tube hav .ing alternating current of one frequencyimpressed on its gridcircuitwhen the frequency of the voltage impressedon another electrode in the tube varies from equality with that of thevoltage impressed on the grid;

Fig. .2 isa graph representing the variations in grid current of twoelectron tubes when alternating voltages of two different frequenciesare impressed on theirgrid electrodesand an alternating voltage of athird frequency is impressed on the two anodes of said tubes;

Fig.3 is a graph representing the variation of the difference of thevoltages represented by the curves in Fig.2; and

Fig. 4 is a diagrammatic showing of a circuit employing the tubeproperties illustrated in Figs. 1, 2 and 3 to produce a discriminatorvoltage proportional in sign and magnitude to the departures of analternating voltage from a fixed or standing value.

Referring in detail to Figure 1 of the drawing, let us suppose that avoltage of constant fre quency (i. e. a frequency which does not varywith time) is impressed on the plate electrode of an ordinaryhigh-vacuum triode, while the frequency of a' voltage impressed on thegrid electrode is gradually varied. If the differencefrequency betweenthe two alternating voltages just mentioned is plotted as abscissa andthe grid current is plotted as ordinate, a curve of the general natureof A will be found to result. Conversely, if the plate current of thetube is measured under the .same conditions, it will be found to berepresented by a curve like curve A turned upside down. The curve A hasa well-defined minimum when the difference-frequency between the twoalternating voltages is zero.

Referrin now to Fig. 2, the curves A and B respectively represent thgrid current or plate current variations of two different tubes,respectively having two different ranges of variable frequenciesimpressed on their grid electrodes and two different constantfrequencies impressed on their plate electrodes. It will be readilyapparent to those skilled in the art that, if the difference betweencurves B and A of Fig. 2 be taken, a current like Fig. 3 will resultwhich is equal to zero for the frequency corresponding to theintersection between the two curves A and B, and which has a straightline relationship for a considerable range of frequencies on each sideof said intersection. Since the currents being plotted are directcurrents, it will be evident that the said difference current is atypical discriminator output current varying with a straight-linerelationship from positive values for frequencies below thatcorresponding to said intersection to negative values for frequenciesabove that intersection.

A circuit well adapted to make use of the general principles justdescribed appears in Fig. 4, in which a source of alternating voltage,which is to be discriminated as to departures from a constant value in,appears at l. Coupling devices 2 and 3 impress this voltage on tubecircuits connected to the anodes respectively of two tubes 4 and 5,which may, for example, be tricdes of the ordinary high-vacuum type.Anode voltage for the tube A is impressed from the positive terminal tof a direct-current source having its negative terminal grounded, andanode voltage for the tube is impressed from the positive terminal '1 ofa similar direct-current source having its negative terminal grounded.The cathodes of the tubes 4 and 5 are likewise grounded and are,respectively, connected to the positive terminals 5 and 7 by bypasscondensers 8 and 9. The anode circuit of the tube 4 comprises anantiresonant c'xcuit consisting of an inductor ii shunted by a capacitorl2, and similarly the anode circuit of the tube 5 comprises anantiresonant circuit consisting of the inductor l3 shunted by acapacitor l4. These circuits may be tuned to two different frequencieswhich may constitute, respectively, the frequencies f1 and f2 of Fig. 2.One convenient form of carrying out the arrangement is to have theinductors ii and it constitute the secondary windings of thetransformers 2 and 3. The grid circuit of the tube 4 comprises apiezo-electric crystal i5 shunted by an inductor it in series with theresistor El and having a natural frequency f1 equal to that of theanti-resonant circuit Ii, i2. It will be recognized by those skilled inthe art that the tube 4 is connected with the crystal i5 in anoscillator circuit.

Similarly, the tube 5 has its grid electrode connected to its cathodethrough a pieZo-electrie crystal l8 shunted by an inductor it? in serieswith the resistor 2!. The crystal l8 should have a natural frequencyequal tojz to which the net 63-44 is tuned. A lead wire 22 is tapped offfrom a convenient point on the resistor El, and a lead wire 23 simi-arlytapped from a point on the resistor 2 i, such that the voltages thusimpressed on leads 22 and 23 are of equal magni tude. The terminals ofthe leads 22 and 23 constitute the discriminator output circuit andtheconnections are such that the lead 22 will be of the same potential asthe lead 23 when the frequency produced by oscillator 5 corresponds tofo the intersection between the curves A and B in Fig. 2. The lead 22may be made positive to lead 23 when the frequency of the oscillator iis below that intersection frequency, and will be negative relative tolead 23 when the frequency of oscillator l is above that intersectionfrequency. It will be noted that the middle branch of the cur e in Fig.3 is the algebraic difference of the portions of curves A and B whichlie between the limitations f1 and f2 and that while the portions ofboth curve A and curve B between th se limitations are slightly curvedthroughout their length, their curvatures are so distributed that theiralgebraic difference is substantially rectiiinear over a considerablerange on each side of the center frequency in. Thus by employing thecircuits of tubes l and 5 in conjunction with each other, an outputvoltage Eis obtained whic is more nearly rectilinear over its workingrange than could possibly be obtained were either one of these tubesomitted from the system. Over a range corresponding to the rectilinearportion of Fig. 3, the potential difference between leads 22 and 23 isthus proportional to the difference frequence of sources 1 and 4-5.

The output voltage from the leads 22 and 23 may be employed for any ofthe purposes filled by discriminator circuits. It may, for example,control a reactance tube which changes the tuning of the oscillator i tobring it back to its original value whenever it departs from thefrequency corresponding to the intersection between curves A and B inFig. 2. Similady, the output voltage from the terminals of leads 2;? and23 may control a direct-current motor which moves a tuning element inthe frequency-deter mining circuit of oscillator l in such a sense as torestore its frequency to that corresponding to the intersection betweencurves A and B in Fig. 2 whenever it departs from that value. To takeanother instance, the output from the leads 22 and 23 may be employedlike other discriminators to detect or demodulate frequency-modulatedsignals in a radio receiver. Such use of discriminator output voltagesis well known in the radio art and requires no detailed descriptionhere.

While I have described the tubes 4 and 5 as two separate triodes, itwill be evident to those skilled in the art that they may comprise tubeshaving a larger number of screening and control electrodes; and, moreparticularly, that instead of employing two separate tubes, theantiresonant circuits ii and I2 may, respectively, be connected to twoelectrodes in a tube having a single cathode, and the piezo-electriccrystals l5 and it be connected to extra grid electrodes in the sametube. In short, the aforesaid anodes and control electrodes may compriseso-called grids in multi-electrode tubes, such as pentodes, heptodes andthe like.

Similarly, while I have described the leads 22, 23 as being tapped offresistors in the control grid circuits of the tubes t and 5, it will berecognized by those skilled in the art that such voltages may be tappedoff appropriate resistors in the cathode leads or the anode leads of theabove-described tubes. By employing tubes having electrodes designed forthe currents and the voltages involved, the voltages impressed on theleads 22 and 23 can be made substantially as high as desired. Likewise,any type of crystal oscillator circuit or other oscillator circuit offixed frequenc can be employed in place of the Pierce oscillatorcircuits shown in Fig. 4.

The curves A and B depend for shape on the same factors that determinethe Q value of their resonant control circuits; hence, the slope of therectilinear portion of the curve in Fig. 3 may be made as steep asdesired by employing resonators of a high equivalent Q value. Sincepiezo-electric crystals normally have a high equivalent Q rating, theuse of piezo-electric crystals at 15 and i8 results in a desirably-steepdiscriminator output voltage with vigorous corrective response to anydeparture of oscillator I from the desired frequency value.

I claim as my invention:

1. A frequency discriminator comprising a first pair of cold electrodeseach cooperating with an electron-emissive cathode and respectivelyimpressed with one of two voltages of different frequencies, a secondpair of electrodes respectively associated with said first pair ofelectrodes and impressed with an alternating voltage of frequencyintermediate between the said frequencies, an electrode from the firstpair cooperating with an electrode from the second pair to constitute acooperating set to produce and regulate current fiow from each saidcathode to one of said cooperating electrodes, and means for buckingcurrent conducted from one of said cold electrodes against currentconducted from another cold electrode in the same pair.

2. A frequency discriminator comprising a first pair of cold electrodeseach cooperating with an elcctron-emissive cathode and connected theretothrough a piezo-electric crystal shunted by an inductor in series witha, resistor, one said crystal having a different resonant frequency fromthe ing a voltage having a frequency intermediate 7 between those ofsaid piezo-electric crystals on each said anti-resonant circuit andterminal leads connected respectively to said resistors to buck againsteach other voltage drops thereon.

3. A frequency discriminator comprising a pair of three-electrode tubeshaving their cathodes connected together, the control electrode of onetube being connected to its cathode through an inductor in series with aresistor and also through a first piezo-electric crystal, the controlelectrode of the other said tube being connected to its cathode throughan inductor in series with a resistor and also through a secondpiezo-electric crystal having a frequency difierent from said firstpiezo-electric crystal, means for connecting the anode of said one tubeto its cathode through a direct-current voltage source and an anti!resonant circuit tuned to the same frequency as said firstpiezo-electric crystal, means for con necting the anode of said othertube to its cathode through a direct-current voltage source and a secondanti-resonant circuit tuned to the same frequency as said secondpiezo-electric crystal, means for impressing on both said anti-resonantcircuits a voltage having a frequency substantially midway between thoseof said piezo-electric crystals, and a line having two sides connectedrespectively to points on the two said resistors.

SHELDON I. RAMBO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,105,096 Peterson Jan. 11, 19382,280,525 Hunt Apr. 21, 1942 2,361,625 Hansell Oct. 31, 1944 2,369,954Downey Feb. 20, 1945 2,373,616 =Sziklai Apr. 10, 1945 2,379,764 ThomasJuly 3, 1945 2,425,981 Bard et al Aug. 19, 1947

